Synaptic physiology of spinal motoneurones of normal and spastic mice: an in vitro study.

Publisher Website
Google Scholar
Abstract
1. Spinal cord reflexes have been examined in a preparation of the mouse spinal cord maintained in vitro. Responses of the motoneurone population of normal and spastic mutant mice to stimulation of a segmental dorsal root were compared. In the normal spinal cord, a monosynaptic response with very little polysynaptic excitation was typical followed by a depolarizing wave on which asynchronous compound action potentials were superimposed. In some spastic cords, an oscillating depolarizing wave was seen, lasting up to 500 ms. The stimulus range from threshold to maximal response was the same for the normal and mutant. The dorsal root reflex (d.r.r.) and dorsal root potential (d.r.p) were prominent in both normal and mutant, and no consistent difference could be identified. 2. Intracellular recordings were made from motoneurones using electrodes filled with potassium acetate. Mean resting potentials and input resistances were not significantly different in mutant and normal mice. The voltage-dependent conductances, seen as the after-depolarization and after-hyperpolarizations following antidromic action potentials and the responses of motoneurones to depolarizing current injection were similar in both populations. 3. The synaptic responses of motoneurones following stimulation of the segmental dorsal root were clearly abnormal in the mutant. In the normal mice, a monosynaptic excitatory post-synaptic potential (e.p.s.p.), seen at low stimulus intensities, was followed at higher stimulus intensities by polysynaptic activity lasting up to 100 ms, which rarely reached threshold for action potential discharge. In the mutant mice, the monosynaptic response was typically followed by depolarizing synaptic responses which often evoked action potentials before the monosynaptic response reached threshold. At higher stimulus intensities, the monosynaptic response was followed by at least one and often multiple action potentials generated on prolonged depolarizing synaptic activity. 4. When cells were impaled with potassium-acetate-filled electrodes, very little spontaneous synaptic activity was seen in either normal or mutant mice. Spontaneous depolarizing post-synaptic potentials (p.s.p.s) were prominent in normal motoneurones when potassium chloride was used to fill electrodes and were increased in amplitude by ionophoresis of chloride into the cells. Under these conditions stimulation of a ventral root evoked a depolarizing p.s.p. and the Renshaw i.p.s.p. reversed. The spontaneous p.s.p.s. were blocked by ionophoresis or bath application of the glycine antagonist strychnine. In mutant motoneurones neither spontaneous nor a recurrent evoked depolarizing p.s.p could be demonstrated following intracellular ionophoresis of chloride. 5. Synaptic responses of normal motoneurones to orthodromic stimulation were profoundly altered following intracellular ionophoresis of chloride. The monosynaptic response was now followed by a depolarizing wave which evoked multiple action potentials, increasing with increasing stimulus intensities to last up to 1 s. In spastic cords recordings made with electrodes filled with potassium chloride were indistinguishable from those made with electrodes filled with potassium acetate. 6. The abnormal motor output of the spastic mutant mouse is thus related to a dramatically reduced chloride-dependent, glycine-medicated synaptic conductance. This is consistent with the hypothesis that the mutation affects the expression of the glycine receptor protein, the associated chloride channel or the interaction between them.